Storage Foundation Software stack

Abstract

Veritas Volume Manager operates in both user space and kernel space: User mode programs like vxassist interact with other user space programs like vxconfigd to create, modify, and delete volumes and other virtual objects. This is done during preparation of the volumes (i.e. creating them or starting them after importing a disk group) as well as during maintenance on the volumes (like resizing them or handling snapshots) or diagnosis (like running the vxprint and vxdisk list commands). In a running system where all necessary volumes have been started the only parts of VxVM actually needed are the device drivers that map volume regions to physical disk regions by applying the plex mapping table to I/O request (vxio) and for multiplexing disk paths (vxdmp). Of course it would be a very radical approach to try it, and full functionality could not be guaranteed, but it is in fact possible to run a UNIX machine using VxVM volumes without a single Volume Manager process running. It would not be possible to do any kind of maintenance on the volumes, like starting or stopping them, or importing or deporting a disk group, but those volumes that have already been started would be running perfectly well. This was a deliberate design decision by the developers because it eliminates a possible single point of failure: If a user process was necessary to enable volume I/O, then a system might be rendered unusable if this process crashed or was stopped by the user. Device drivers, unlike user processes, cannot be killed or unconfigured "against their will". Having the system be independent from user space processes for the bulk of the work (user I/O) was therefore a wise thing to do.